rear wheels engage with a thunk

The other day I decided to venture out in my 04 outback with an automatic without shoveling my driveway. there was about 8 or 10 inches of snow. I made it out just fine. On the way back in I was
going slow, and got high centered. I waited a few minutes for the exhaust to melt the snow down, then tried to back out. The car struggled back a few feet, then once the back wheels had made it to the cleared street I thought I'd be fine. they were on no snow, but the front wheels wer spinning and throwing up snow and the car wasn't moving. I revved the engine a little to see if I could dig down through the snow to pavement, and I heard a thunk from the rear and the car backed up. Was this a clutch pakc suddenly engaging or something? Why hadn't it engaged before that?
I've owned two second gen legacies with manuals and have never ever had problems in snow. I've heard the center diff is different between the two transmissions. Is this the reason for my near stuckedness?
What's going on here? Is this normal, or is the 04 not quite working properly?
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this is very interesting. I want to read other responses, but have 2-3 comments myself. I'd want to check the U-joints for one. Also, the transmission fluid level and condition.
I have a very vague memory of reading that, when in reverse(and maybe when 1st is 'selected' with the shifter), the TCU/ECU/'computer' always has AWD fully engaged - so, it DOES seem that something was 'stuck' related to the clutch pack.
curious
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It does sound like the clutch pack did lag in engaging. One of a few things that will cause this is a grooved clutch pack carrier (happens to motorcycles, too). The clutch plates get stuck 'together' or 'apart', then they shift into place.
Failing Duty C solenoid will do this too.
Do some full-lock circles in a dry parking lot and see if you have any torque-bind symptoms.
Dave
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I did some full lock turns with no apparent bind. Then again, I'm assuming that bind would show up as either tire squeal or some jerkiness to the drivetrain. Neither of those. Would the clutch pack carrier be located in the housing for the rear diff? As in, could I access this part by taking off the driveshaft, then dropping the housing that holds on the rear diff? Then again, I'm not sure if the auto has that separate housing like the manual of the previous generation legacies does.
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This is normal. We often run Subaru's on a rack to 50-60 mph testing wheel bearings; we have to rev them up and down to force all the wheels to engage. The "clunk" can be quite noticeable when there is no load on the tire(s) .
weelliott wrote:

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